Title of Invention

AN IMPROVED STEEL BAR COMPOSITION AND PROCESS FOR PRODUCING STEEL BARS FOR CONVERSION INTO AUTOMOBILE GEARS

Abstract

A substitute steel to SAE 8620 has been commercially developed based on indigenously available alloying elements like Mn, Si and Cr The substitute steel contains only one third of molybdenbum and half of the nickel contents of conventional SAE 8620 steel The core and case hardenabilities of the steel were found to be quite comparable with those of SAE 8620 steel The steel exhibited a yield stress of 75 kgf/mm2 and tensile strength of 95 kgf/ mm2, with 20% elongation The charpy impact value was found to be 3 5 kgM The substitute steel also offered better resistance to tempering than SAE 8620 steel at all the temperatures up to 600 °C as demonstrated by mechanical properties and micro structure Tata Mercedes Benz gears were successfully made from the steel The case and core hardness and the micro structure of the gears were found to be similar to those of SAE 8620 gears.

Full Text

2 The present invention relates to an improved process for producing steel bars for conversion into automobile gears The invention relates more particularly for producing low cost steel round bars containing reduced alloying elements like Mo and Ni which are relatively expensive. In the existing process for producing round steel bars for conversion into automobile gears a relatively increased proportion of Mo and Ni alloys, such as, 0.15 to 0.25% by weight of Moana 0.40 ton 0 70% by weight of Ni, is added in the composition of the steel bars with consequent rise in the cost of production The objective of the present invention is to provide a process for producing steel bars for conversion into automobile gears at a relatively low cost by reducing the content of Mo and Ni alloys in the composition thereof Extensive laboratory-scale and mill-scale trial productions of steel containing reduced Mo and Ni alloys have been made followed by evaluation of the mechanical properties of the steel produced, for optimization of the chemical composition of steel with reduced Mo and Ni content, which is suitable for conversion into automobile gears of required mechanical properties and performance characteristics. The optimized composition of the steel bars produced in the invented process is given in Table I which also includes the composition of the steel bars produced in the existing process The mechanical properties of the steel bars produced in the invented process are presented in Table II The invention is described fully and particularly in an unrestricted manner with reference to the accompanying drawings in which Figure 1 is a flow chart illustrating the main steps followed in the invented process Referring to Fig 1, the invented process comprises the following steps in sequence (i) molten steel of required chemical composition is produced in an electric arc furnace using raw materials such as steel scrap, lime iron ore and ferroalloys , (ii) molten steel is tapped in a preheated ladle at a temperature of 1630 ± 20°C ; (iii) molten steel in the ladle is cast as ingots weighing 2.35 tonne each ; (iv) ingots are reheated and soaked at 1250 ± 20°C for 2 to 3 hours , (v) soaked ingots are rolled into 130 mm square billets , (vi) billets are reheated and soaked at 1250 + 20°C for 1 to 2 hours , (vii) soaked billets are rolled into 63 mm diameter round bars, (viii) round bars are cooled in natural air The round bars are converted into automobile gears by reheating, forging into discs of 140 ram diameter and 28 mm thickness, heat treatment, hobbing into gears, carburising and finishing treatment, From Table I, it is noted that the content of Mo is reduced to 0.07-0 12% in the invented process from 0 15-0 25 in the existing process, and that of Ni is reduced to 0 20-0 35% in the invented process from 0 40-0 70% in the existing process, because of which the production cost of steel bars is appreciably lower in the invented process compared with the existing process. From Table II, it is noted that the mechanical properties of the steel bars produced in the invented process and also the hardenability thereof are found to be comparable with those of the steel bars produced in the existing process 4 TABLE- I Composition (Wt, %) ELEMENTS INVENTED PROCESS EXISTING PROCESS RANGE TYPICAL RANGE C 0.18-0.23 0.22 0.18-0.23 Mn 0.95-1.15 1.12 0.70-0.90 Si 0.30-0.45 0.45 0.20-0.35 S 0.020 max. 0.01 P 0.020 max. 0.02 Cr 0.50-0 70 0.62 0.40-0.60 Mo 0.07-0.12- 0.07 0.15-0.25 Ni 0 20-0.35 0.30 0.40-0.70 Fe Balance Balance Balance TABLE - II Mechanical properties * of steel bars produced in the invented process SPECIFIED VALUES. TYPICAL VALUES Yeild strength (Mpa) 490 (min.) 736 UTS (Mpa) 675 (min.) 929 Elongation (%) 12 (min.) 21 (GL = 20 mm) Charpy impact toughness at RT - 35 (Joule) * Mechanical properties are for 50 mm round bar carbunsed at 925°C for 1 hr., quenched in oil and tempered at 190°C for 2 hrs. We Claim 1 An improved process for producing steel bars for conversion into automobile gears comprising the following steps in sequence (i) molten steel of required chemical composition is produced in an electric arc furnace using raw materials such as steel scrap, lime, iron ore and ferroalloys , (ii) molten steel is tapped in a preheated ladle at a temperature of 1630 ± 20°C , (iii) molten steel in the ladle is cast as ingots weighing 2 35 tonne each (iv) ingots are reheated and soaked at 1250 ± 20°C for 2 to 3 hours , (v) soaked ingots are rolled into 130 mm square billets , (vi) billets are reheated and soaked at 1250 ± 20°C for 1 to 2 hours , {vii) soaked billets are rolled into 63 mm diameter round bars, (viii) round bars are cooled in natural air , characterized in that the chemical composition of the steel bars produced is (by weight %) C-0 18 to 0 23, Mn-0 95 to 1 15, Si-0 30 to 0 45, S-0 020 max , P-0 020 max , Cr-0 50 to 0 70, Mo-0 07 to 0 12, Ni-0 20 to 0 35, Fe-balance 2 The process as claimed in claim 5, wherein the chemical composition of the steel bars produced is (by weight %) C-0 22, Mn-1 12, Si-0 45, S-0 01, P-0 02, Cr- 0 62, Mo-0 07, Ni-0 30, Fe-balance 3 Steel bar composition especially for conversion into automobile gears and a process for producing the same substantially as hereindescribed and illustrated with reference to the accompanying examples Dated this 27th date of December 1999. A substitute steel to SAE 8620 has been commercially developed based on indigenously available alloying elements like Mn, Si and Cr The substitute steel contains only one third of molybdenbum and half of the nickel contents of conventional SAE 8620 steel The core and case hardenabilities of the steel were found to be quite comparable with those of SAE 8620 steel The steel exhibited a yield stress of 75 kgf/mm2 and tensile strength of 95 kgf/ mm2, with 20% elongation The charpy impact value was found to be 3 5 kgM The substitute steel also offered better resistance to tempering than SAE 8620 steel at all the temperatures up to 600 °C as demonstrated by mechanical properties and micro structure Tata Mercedes Benz gears were successfully made from the steel The case and core hardness and the micro structure of the gears were found to be similar to those of SAE 8620 gears.

Documents:

01010-cal-1999-abstract.pdf

01010-cal-1999-claims.pdf

01010-cal-1999-correspondence.pdf

01010-cal-1999-description(complete).pdf

01010-cal-1999-drawings.pdf

01010-cal-1999-form-1.pdf

01010-cal-1999-form-18.pdf

01010-cal-1999-form-2.pdf

01010-cal-1999-form-3.pdf

01010-cal-1999-pa.pdf

1010-cal-1999-granted-abstract.pdf

1010-cal-1999-granted-claims.pdf

1010-cal-1999-granted-correspondence.pdf

1010-cal-1999-granted-description (complete).pdf

1010-cal-1999-granted-drawings.pdf

1010-cal-1999-granted-examination report.pdf

1010-cal-1999-granted-form 1.pdf

1010-cal-1999-granted-form 19.pdf

1010-cal-1999-granted-form 2.pdf

1010-cal-1999-granted-form 3.pdf

1010-cal-1999-granted-letter patent.pdf

1010-cal-1999-granted-others.pdf

1010-cal-1999-granted-pa.pdf

1010-cal-1999-granted-reply to examination report.pdf

1010-cal-1999-granted-specification.pdf